A display device in which a reflection type element and a light emission type element are combined has been proposed (Patent Document 1). The reflection type element is used in bright environments and the light emission type element is used in dark environments, so that it is possible to achieve high display quality independent of environment light and to provide a low power consumption display device.
A technique for using an oxide semiconductor transistor (hereinafter referred to as an OS transistor) for a display device such as a liquid crystal display or an organic electroluminescent (EL) display has been proposed. The refresh frequency at the time of displaying still images is reduced because an OS transistor has an extremely low off-state current, resulting in reduction in power consumption of liquid crystal displays or organic EL displays. Such a technique has been disclosed (Patent Document 2 and Patent Document 3). Note that the above-described technique for reducing the power consumption of the display device is referred to as idling stop or IDS driving in this specification.
An example in which an OS transistor, which has an extremely low off-state current, is used in a nonvolatile memory device has been disclosed (Patent Document 4).